The present invention generally relates to computer systems and more particularly to a computer system having a power supply including a transmission line stub coupled to a processor chip.
Computers, including personal computers, servers, workstations, etc., are continually being improved with more processing power and memory. Processing speed has been increased well above 100 megahertz. Processing power improvement involves such speed improvements as well as use of more computing devices, such as digital signal processors and programmable logic cores, which may be included in computers. Nearly all such improvements cause the total power used by a computer to increase and generate more heat which must be removed to avoid damage to the circuits.
One approach to increasing processing power without overheating circuits has been to operate computer chips at lower voltages. At one time, the standard DC power supply for computers was five volts DC. Circuits have since been developed which operate at lower voltages including 3.3 volts, 1.8 volts and 1.2 volts. With each voltage reduction, computer performance has been improved, while power dissipation has been controlled.
At the low operating voltages, it becomes more important to avoid AC noise on the DC power supply busses and connections to the functional circuits. There is little margin for error at the low voltages. An internal source of AC noise on DC power supply lines is the switching transients generated by the chips themselves, and particularly by the output drivers which drive signals from each chip to other chips, signal busses, etc. Transients generated on a given chip will appear on that chip's own power inputs and can affect all circuits on the chip. It is important that the AC impedance of the DC power supply be as low as possible at the power supply inputs of processor chips to minimize noise.
It is well known to use capacitors to reduce the AC impedance of DC power supplies. Such capacitors may be placed on a computer motherboard near the power supply pins of the processing chips. Capacitors may also be placed on chip packages, especially on microprocessor packages. It is known that essentially all capacitors are somewhat inductive and have a resonance frequency at which they have low AC impedance. Above the resonance frequency, the parasitic inductance causes the impedance of a capacitor to increase. Smaller value capacitors have higher resonance frequency and are often used in parallel with larger value capacitors to remove AC noise from DC power supply lines over a broader frequency band. As circuit operating speed has increased, the frequency of power supply noise sources has also increased. Adding capacitors to a motherboard or package is expensive in terms of the cost of the capacitors themselves, the space required on the motherboard and the assembly time and expense required for additional components.
It would be desirable to provide a DC power supply system with low AC impedance for computer systems which minimizes the need for capacitors or at least for high resonance frequency capacitors.